Ring Laser Gyros having two or more beams of coherent light propagated in opposite directions around the laser ring cavity are well known in the art. See, U.S. Pat. No. 3,382,758 issued May 14, 1968, entitled "Ring Laser Having Frequency Offsetting Means Inside Optical Path" by Wang. The Ring Laser Gyro is a device for measuring rotation about an axis by means of counter propagating coherent light beams which travel differing effective path lengths in the clockwise and counter clock wise directions. The Ring Laser typically consists of four mirrors (one partially transparent) arranged in a square or polygonal shape defining a laser cavity and one or more laser tubes inserted into the cavity to provide light amplification and a coherent light source. Once laser oscillation occurs in the system at resonant frequencies, the difference in the length of the pathways traversed by the counter propagating laser beams result in a difference or beat frequency which is sensed by a photodetector and amplified by an amplifier. The beat frequency is a result of optically heterodyning the counter propogating beams.
The prior art recognized the sensitivity of the ring laser to factors like the earth's magnetic field, the direction of propagation of the beams, and the ellipticity of the polarized light forming the beams when precise rotational measurement is needed. The prior art has attempted to eliminate sensitivity of the ring laser gyro to the magnetic field of the earth in U.S. Pat. No. 4,213,705, issued July 22, 1980 to Virgil Sanders, entitled "Four Mode Zeeman Laser Gyroscope With Minimum Hole Burning Competition."
Ideally a planar two mode ring laser gyro should have linearly polarized beams. Mirror birefringence, out-of-planeness, relative reflectivity of the mirror of perpendicular and parallel polarized light, and the Faraday effect are all factors which cause the gyro system to deviate from this ideal. Non-planar multimode ring laser gyro systems provide an output signals at the partially transparent mirror which combines circularly polarized laser beams. The output signals provided would yield a most accurate measurement if they could pass through this partially transparent mirror relatively undistorted.